This project is directed to the development of utilization of the ciliated protozoa as instruments of scientific analysis. It focuses on the Tetrahymena pyriformis complex that consists of at least 18 genetically isolated gene pools. All the species are used in comparative and evolutionary studies and intensive genetic analysis is applied to two species with apparently very different genetic economies: T. theromphila and T. pigmentosa. Within the Species we are concerned with the regulation of genetic function, and particularly with the changes in genetic function and organization associated with macronuclear development. We are particularly concerned with mating types, antigens, and isozymes.